Sweetening process



Oct. 1951 H. A. RICARDS, JR, ET AL 2,570,277

SWEETENING PROCESS Filed Feb. 24, 1949 SODIUM 122x325: 9\ W F E E.l jSTAGE 2 2o l SPENT CAUSTIC I SODIUM L HYDROXIDEN 15 v ,1?

.. Y (1(0- STAGE 5 30 T CoO- (Inventors 7W abhor-neg improved sweeteningprocess. more particularly concerned with the removal of ing oil boilingranges.

Patented Oct. 9, 1951 *tiNlTED STAT-ES PATENT OFFICE SWEETENING PROCESS.Harold A.-Ricards, .Jr., Cranfordand James W. RydenvwatchungpN. J.,assignors to Standard Oil Development Company, a corporation of DelawareApplicationFebruary 24, 1949, SerialNo. 78,042

9Claims. (Cl. 19632) Q The present invention is concerned with-an Theinvention is mercaptan compounds, particularly those mercaptan compoundswhich have heretofore been relatively diflicult to remove :fromhydrocarbon fractions especially from thosefractions boiling in themotorfuel, kerosene, diesel oil andheat- In accordance with the presentinvention a feed hydrocarbon oil-characterized by containing mercaptancompounds, various phenolic compounds andother acidic compounds, such aslower" molecular weight-aliphatic acids, is contacted in a seriesoftreating zones wherein a particular arrangement-and sequence of thestages-are employed. In accordance with the broadcast conceptof-thepresent process a feedoil containing mercaptan-compounds iscontacted in a primary zone with a 'relatively concentrated I causticsolution under conditions to remove from the feedoil objectionablecompounds as for example phenolic --compounds. The oil is then contactedin a secondary zone with -a less concentrated caustic solution than thatemployed in the prima-ry-zone and with solubility enhancing agents under"conditions to remove from-theoilthe objectionable mercaptan compounds.

Thisapplication contains material claimed incopending"application SerialNo. 76,802;'fi18d "February 16, 1949 for the same inventors.

Itiswell known inthe-art totreatpetroleum oils -byvarious procedures inorder to remove objectionable compounds, as for'example sulfur compoundstherefrom. For example, it is known. to treat petroleum fractionsboilingin the motor fuel boiling range and in the-general range belowabout '7 F. with various chemicalreagentssuch as sodium or potassiumhydroxide solutions.

Also it has long beenknown-'that 'weaklyacidic materials, suchasmercaptans, are reactive to various degrees with basic materials. One

method for the removal of mercaptans from hydrocarbon streams has beento treat the streams -withareagent which is-insolublein the hydro--carbon stream but which contains alkali type-of material which reactsWith-mercaptans Such treating reagentshave normally been aqueoussolutions. The reaction product is abasic-salt of the mercaptan.Normally such salts exhibit some solubility in the aqueous phase, thissolu- -bility decreasing as the molecularweight or branchiness of thehydrocarbonstructure of the mercaptan-increases. The arthasgenera-llyrecognized inthe treating of hydrocarbon streams:

tan extraction.

25531011 or by oxidation.

One method is to employ steam regeneration for the revivification ofcaustic solution spent in mercaptan removal from hydrocarbon streams.

The steam regeneration is normallycarried out in. a packedtower or atower provided withbubble plates or pierced type tray plate=design.,.The

steam hydrolizes the mercaptide. salts to the corresponding mercaptanswhich are strippedout By the steam vapors and taken overhead. Theregenerator also serves as ameans of adjusting. the

gravity of the. caustic. solution which has been shown. to. be critical.The regenerated caustic-is Withdrawn from the bottom of the.regeneration tower and reused in the extraction stage. of the systemwherein the caustic contacts the hydrocarbon stream through suitablemixing. devices for further extraction of mercaptans byJthe samecaustic.

It has. also been found that certain constituents .will function assolubility enhancing .agents. These constituents tend to improve thepartition coeflicient of the acid mercaptanin. theaqueous phase and toincrease thesolubility ofthemercaptide salts in theaqueous phase,particularly when the mercaptan is of a relatively high molecular.weight. As mentioned, .-mercapt ans are difificult to extract fromhydrocarbonsolutionby straight aqueous alkali solutions. Fhis isparticularly true of the higher molecular'weight mercaptans. .Ingeneral, this is occasioned bythe fact that the two phases arenotmiscibleland that good contact-must be obtained to allow the alkalimetal ion to reactwvitnthe mercaptan hydrocarbon. Furthermore, .once.the reaction has occurred,-it isessential thatif the extraction of themercaptan is tobe accomplished the re- .action product. must be solublein the .aqueous layer. Astpointed out heretofore the higher "themolecular Weight-of the mercaptanthe lowenthe solubility of thecorresponding mercaptidesalt in the aqueous phase. The solubilityenhancing agents tend to increase this solubility and thus increase theeffectiveness of the alkali treating solution.

The exact chemical nature of many of these solubility enhancing agentsis not entirely known. However, it is felt that they comprise the lowermolecular weight aliphatic acids and the various substituted phenoliccompounds. It has been found that phenolic type materials having alkylside chains containing less than about 3 carbon atoms are not effectiveby themselves as solubility promoters and that organic carboxylic acidshaving less than about 3 carbon atoms per molecule are like-wise notvery effective. However, as the length of the side chain of the phenolsincreases or the length of the aliphatic acid hydrocarbon chainincreases, their effectiveness as solubility promoters increasesgreatly. However, the solubility of the acids or phenols themselves inthe caustic solution decreases with increasing hydrocarbon chain lengthand thus limitsthe molecular weight of acid or phenolic type materialwhich can be employed. It is believed that the optimum chain lengthshould be from about 3 carbon to '7 carbon atoms for the acids.

Particularly desirable aliphatic acids are for example, isobutyric acid,propionic acid and valeric acid. Other satisfactory acids are normalbutyric acid, caproic acid, alpha ethyl butyric and alpha methyl butyricacid. These acids may be substituted with an OH group or with a halogen.Various alcohols are also satisfactory, as for example, methyl alcoholand ethyl alcohol. Ethylene glycol, as well as, propylene glycol andother poly hydroxy alcohols may also be used. Naphthenic acids, as forexample dichlorocatechol naphthenic acid is also satisfactory.

A particularly desirable class of solubility enhancing agents to be usedin conjunction with the present invention is a naturally occurring agentwhich is present in petroleum hydrocarbons. This is segregated frompetroleum hydrocarbons by treating the same with a relatively weakcaustic solution.

The fresh sodium hydroxide employed for the segregation of naturallyoccurring solubility enhancing agents should have a Be. gravity belowabout 10 and preferably have a gravity in the range from about 1 to 5".Thus, the concentration of the sodium hydroxide in the .fresh causticsolution should be below about 7% and is preferably in the range fromabout .7 to 3 /2%. When the concentration of the caustic solution isspecifled in the present invention, it is meant for example, the gramsof sodium hydroxide present in a hundred grams of mixture comprisingsodium hydroxide and water. By utilizing a solution of this character ithas been found that the most effective solubility enhancing agents canbe readily removed from the oil. Furthermore, it has been found thatwhen utilizing a caustic solution having these critical concentrationsthe various phenolic compounds and other constituents which apparentlygreatly adversely affect the efiiciency of the solubility enhancingagents are not picked up by the caustic solution but remain in the oilstream being treated.

Thus, a very sharp selective separation is made between the desiredsolubility enhancing agents which are picked up in the caustic solutionand the other acidic constituents which adversely affect the efficiencyof the solubility enhancing agents. While a once through operation maybe employed'with respect to the caustic stream, it is preferred toemploy at least a partial recircula tion of the caustic stream withdrawnfrom the initial zone. The spent caustic stream withdrawn from thecontacting zone and not recirculated is controlled so as to be spent inthe range from about 30% to 100%. A preferred method of operation is tocontrol the various factors so that the spent caustic stream withdrawnand not recirculated is spent to a degree in the range from about 30% to50%. It is to be understood that the term spent when used in the presentinvention means that percentage of sodium ion which has reacted withorganic carboxylic acids, as for example with aliphatic acids, toproduce sodium salts of these acids. In the operation, upon initialcontact a portion of the sodium might react with the various phenols toform sodium phenolates. However, upon recirculation the phenolates arereconverted to the phenols and are displaced from the aqueous solutionwith the resulting formation of sodium salts of aliphatic acids. Thus,the amount of sodium not spent may be available either in the form ofthe hydroxide or as a sodium phenolate.

The concentration of the caustic employed in the present invention withrespect to the segregation of naturally occurring solubility enhancingagents is a concentration at which effective results are secured. It isobvious that stronger caustic solutions could be initially used, as forexample 10 B. or higher and somewhat similar results obtained byrecirculating the spent caustic until from about to 100% is spent, thatis, until 70% to 100% of the sodium ion is reacted to form sodium saltsof aliphatic acids. When employing caustic solutions having greaterconcentrations than about 7 B., it is preferred that the causticsolution withdrawn from the initial stage be spent in excess of about90%. As pointed out heretofore, the broad concept of the presentinvention is to treat a feed oil in an initial stage of a two-stageprocess with a relatively strong caustic solution andthen to treat theoil in a secondary stage with a less concentrated caustic solution inthe presence of solubility enhancing agents.

The process of the present invention may be readily understood byreferring to the drawing illustrating an embodiment of the same. Referring specifically to the drawing a feed oil which for the purpose ofillustration is assumed to be a petroleum fraction boiling in the rangefrom about to 420 F. is introduced into a hydrogen sulphide treatingzone I by means of feed line 2. In zone i the feed oil is processed in amanner adapted to remove substantially completely the hydrogen sulphidefrom the feed oil. The treating reagent, if one be employed, isintroduced into zone I by means of line 3 and withdrawn by means of line4. It is to be understood that if the feed oil is free of hydrogensulphide, this preliminary treatment in zone I may be dispensed with. Avery desirable method of removing hydrogen sulphide is by a distillationoperation. This is usually accomplished in a stabilization anddebutanization operation wherein the hydrogen sulphide is removedoverhead with the propane and lighter constituents.

The treated oil is removed from stage i by means of line 8 and isintroduced into an initial caustic treating stage 20 wherein the same iscontacted with a relatively concentrated sodium hydroxide solution. Thesodium hydroxide solution is introduced into stage or zone 20 by, meansof line 9 and is withdrawn by means of line ll.

50. and withdrawn by meansof line IS. The finished oil having arelatively low copper number is arrears The treated oil is removed fromstage 20 by means of line I2 and isintrOduced into a secondary caustictreating stage '30, wherein the same'is contacted with a sodiumhydroxide solution having a'c'onc'entration less than the solu- 1 tionemployed in the initial stagean'd containing solubility enhancingagents. The sodium hydroxide solution is introduced into stage 39 bymeans of line I5 and the spent solution withdrawn by meansof line 22.

line I6 and is preferably water Washed in zone Water is introduced bymeans of line I1 withd'rawn'from zone '50 by means of line l9 andfurther refined or handled as desired.

The spent caustic solutionre'moved'fro'm initial caustic treating stagemay be withdrawn are reconverted tomercaptans by hydrolysis and areremoved overhead by means of line 24. Steam is introduced into zone 60by means of line 23. The regeneration operation conducted in zone 60also is controlled to adjust the gravity of the regenerated solution.The regenerated caustic may be removed from the system by means of linebut is preferably recycled in a manner as hereinafter described tosecondary zone by means of lines 26 and I5.

The invention is broadly directed toward a two stageprocess wherein afeed oil is contacted with a relatively concentrated caustic solution inan initial stage and with a lessconcentrated caustic solution in asecond stage. The treatment in the initial stage serves to removevarious constituents as for example various low molecular weightphenolic compounds and related constituents which would otherwise serveto greatly adversely aifect the efiiciency of the solubility enhancingagents used in conjunction with the caustic treating solution employedin the second stage.

The invention may be utilized for the removal of mercaptan compoundsfrom any type of feed oil. It is particularly adapted however for theremoval of mercaptan compounds from petroleum hydrocarbon fractionsboiling below about 700 F. as for example for the removal of mercaptancompounds from hydrocarbon fractions which boil in the motor fuel,kerosene, diesel oil and gas oil boiling ranges.

While the present invention may be used in the treatment of any type offeed oil, as for example for the treatment of a mixed cracked and virginfraction, it is apparent that the present invention is particularlyadapted for the processing of cracked fractions derived from aromatictype crudes containing naphthenic acids. It is desirable in accordancewith the present invention that hydrogen sulphide be substantiallycompletely removed from the feed oil in a preliminary treating zone.

The strength of the fresh caustic added in the initial caustic treatingstage is preferably in the range from about 24% to 30% NaOH. Herealthough a once through operation may be utilized, it is preferred toemploy at least partial recycling under conditions so that the totalcaustic stream utilized in the treatment of the oil in the secondarystage does not have a concentration less The treated oil is'withdrawn'from tertiary stage 30 by means of than about 12% t'o 15%NaOI-I. If thelsperit eaustic stream has approximately these critical minimum concentrations a portion of 1 the stream is with'drawn from thesystem while a :portion of the stream may be recycledand mixed withfresh caustic solution having a concentration as specified above. 'Byoperating in this manner the phenolic and other acidic constituentswhichadversely affect the efiici'ency of the solubility enhancing agentsin the secondary stage :for the removal of mercaptans are effectivelyremoved from the oil. The amount of caustic solution employed by volumebased upon the oil may vary appreciably. However, it is preferred thatin the initial stage the amount of causticsolution employed based uponthe oilbe sufiicient'to'obtaingood mixing and remove desiredacidicconstituents, which by volume'is in the range from about 5%to 50%.

The concentration .of the caustic solution employed in the second stageis less thanthe concentration of the caustic used to contact the oil inthe initial stage. As previously mentioned the concentration of thecaustic solution employed in the initial stage is preferably intherange-from about 12% to 15%. The concentration of the caustic employedin the secondary stage for any particular operation is less than thesespecified concentrations for the initial stage, and is in the generalrange from about 9% to 12%. It is preferred that the concentration ofthe caustic employed in the secondary stage be fromabout 1% to 3% lessthan the caustic :employed in "the initial stage. I

These desired concentrations of the caustic for utilization inthesecondary stage may be secured by controlling the amount'of freshcaustic added and the extent to which the spent caustic is regenerated.i

The amount of caustic utilized in the secondary stage is from about 5 to50% by volume based upon the volume of oil being treated. Theconcentration of the solubility enhancing agents present in the causticsolution introduced into the tertiary stage is preferably in the rangeso that from about 25% to 75% of the total sodium or other alkali metalis present as a sodium salt.

By operating in the manner described and controlling the concentrationof the caustic stream, phenolic and other acidic constituents which werenot removed from the oil in the initial stage will not be removed fromthe oil in the secondary stage. Thus, these objectionable phenoliccompounds and other acidic constituents will not build up or accumulatein the circulating caustic stream of the secondary stage with aresulting impairment of efficiency with respect to the removal ofmercaptan compounds from th oil stream. The spent caustic stream may berecirculated to a revivification or regeneration unit as hereinbeforedescribed.

It is to be understood that the present invention may be employed in thetreatment of various petroleum fractions other than motor fuels althoughit is particularly adapted for the production of high quality motorfuels. Heating oils, kerosenes, and the like may be treated in a manneras described. Although the invention has been particularly describedwith respect to the use of a sodium hydroxide solution, potassiumhydroxide, and other alkali metal hydroxide solutions may also be used.

Having described the invention it is claimed:

1. Process for the removal of mercaptan compounds from hydrocarbonmixtures containing the same which comprises contacting the hydrocarbonfraction in an initial stage with a caustic solution, removing thecaustic solution therefrom followed by contacting the hydrocarbonmixture in a secondary stage with a caustic solution of at least about 1to 3 per cent lower concentration than that employed in the initialstage and using therewith agents which increase the solubility ofmercaptans and mercaptides in said caustic solution. I

2. The process defined by claim 1 in which at least a portion of thecaustic solution employed in the said initial stage is recycled to thesaid initial stage and in which at least a portion of the said causticsolution employed in the said secondary stage is recycled to the saidsecondary stage after at least partial removal of mercaptans containedin the solution.

3. Process as defined by claim 1 wherein the hydrocarbon mixturecomprises a petroleum hydrocarbon fraction boiling below about 700 F.

and wherein the concentration of the caustic solution employed in theinitial sta is in the range from about 12% to 15% 'andwherein theconcentration of the caustic solution employed in the secondary stage isin the range from about 10% to 12%.

4. Process as defined by claim 1 wherein said agents used in conjunctionwith the caustic in the secondary stage comprise aliphatic acidscontaining from about 3 to 7 carbon atoms in the molecule.

5. Process as defined by claim 1 wherein said solubility enhancing agentcomprises a naphthenic acid.

.6. Process as defined by claim 1 wherein said solubility enhancingagent comprises an alcohol. 7. Process as defined by claim 1 whereinsaid solubility enhancing agent comprises alkali metal salts ofaliphatic acid which have been segregate from a petroleum hydrocarbon.

8. An improved process for the removal of mercaptan compounds from a fed oil which is characterized by Containing hydrogen sulphide andmercaptan compounds which comprises processing said feed oil in atreating zone to substantially completely remove the hydrogen sulphidefrom said feed oil, then contacting said hydrogen sulphide-free feed oilwith a relatively strong caustic solution in an initial caustic treatingstage followed by contacting said oil in a secondary caustic treatingstage with a caustic solution which is at least about 1 to 3 per centless concentrated than that employed insaid initial stage and using inconjunction therewith solubility enhancing agents. j

9. Process as defined by claim 8 wherein the hydrocarbon mixturecomprises a petroleum hydrocarbon fraction boiling below about 700 F.and wherein the concentration of the caustic solution employed in theinitial stage is in the range from about 12% to 15% and wherein theconcentration of the caustic solution employed in the secondary stage isin the range from about 10% to 12%.

HAROLD A. RICARDS, JR. JAMES W. RYDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,935,725 Perl Nov. 21, 19332,317,053 Henderson Apr. 20, 1943 2,431,770 Payne et al. Dec. 2, 19472,462,810 Ferguson Feb. 22, 1949

1. PROCESS FOR THE REMOVAL OF MERCAPTAN COMPOUNDS FROM HYDROCARBONMIXTURES CONTAINING THE SAME WHICH COMPRISES CONTACTING THE HYDROCARBONFRACTION IN AN INITIAL STAGE WITH A CAUSTIC SOLUTION, REMOVING THECAUSTIC SOLUTION THEREFROM FOLLOWED BY CONTACTING THE HYDROCARBONMIXTURE IN A SECONDARY STAGE WITH A CAUSTIC SOLUTION OF AT LEAST 1 TO 3PER CENT LOWER CONCENTRATION THAN THAT EMPLOYED IN THE INITIAL STAGE ANDUSING THEREWITH AGENTS WHICH INCREASE THE SOLUBILITY OF MERCAPTANS ANDMERCAPTIDES IN SAID CAUSTIC SOLUTION.